As is well known, drilling fluid is utilized in well-drilling operations for a number of basic purposes. One purpose is to cool and lubricate the bit and the string. Another is to carry up to the surface the bore hole material which is produced as a result of the drilling operation. A third purpose is to deposit a tough and low-permeability filter cake against the sides of the bore-hole and thus reduce the invasion of the fluids phase into the formation and control fluid losses down hole. A fourth is to overbalance formation pore-pressures with sufficient hydrostatic head in order to control well flowing. A fifth is for control of corrosion of the drill string and bit. A sixth is to buoyantly support the drill string.
Due to geothermal heat in the surrounding formations, the temperature of the drilling fluid can rise as high as 600.degree. F. or more. The pressure of the drilling fluid is a function of depth and density. For very deep wells, the pressure of the drilling fluid at the bottom of the well can be as high as 20,000 psi or more.
It is very important for the drilling fluid not only to deposit a low permeability filter cake on the borehole wall, but one which is sufficiently tough and cohesive to resist erosion by the upwardly flowing drilling fluid in contact with the borehole walls.
The co-pending U.S. application Ser. No. 551,343, filed Nov. 14, 1983, is directed to an apparatus which comprises a main housing, a permeable cylindrical sleeve fixed with respect to the main housing and simulating a rock formation through which a well is drilled, first means defining a compartment exterior to the sleeve, second means for introducing into the interior of the sleeve a solids-containing fluid and for raising the fluid to a pressure higher than the pressure in the compartment, whereby the fluid passes through the sleeve and into the compartment, leaving a filter cake on the inside of the sleeve, and a probe extending through the sleeve and simulating a drill string. The apparatus is capable of depositing a filter cake and of making various measurements under controlled hydraulic regimes.
The present invention pertains to an apparatus for cooperation with the apparatus just described, which allows a determination of the soil mechanics properties of the filter cake, and the profile of filter cake thickness measured circumferentially around the sleeve. The apparatus is also capable of off-centering the probe, which may induce non-uniform filter cake thickness.
In a general way, it is important to be able to determine the effect of various hydraulic regimes on a filter cake in relation to its soil mechanics properties.
As used hereinafter the word "stickometry" will refer to the laboratory measurement of the force required to break a contact probe free of a filter cake, thus modelling a possible down-hole condition. This deserves further explanation.
As used hereinafter, the word "consolidometry" will refer to the rate and degree of consolidation of the filter cake.
When drilling a well, the drill string normally does not remain in an axially centred position with respect to the hole being drilled. Often, the hole will deviate, and the drill string will tend to approach the side of the drill hole. As previously stated, when the fluid phase of the drilling fluid seeps away into surrounding rock strata where the same is permeable, a filter cake of lower permeability is deposited on the sides of the hole, thus cutting down on the loss of fluid. However, in the event that the drill string is stopped when in contact with the filter cake, it will be appreciated that the drill string develops differential pressure sticking because the filter cake masks a portion of the drill string from a balanced hydrostatic condition. The force resulting from this differential pressure sticking can be substantial.
At present, several methods are utilized to try to break the drill string free of its stuck position against the filter cake. One of these is simply to cause a shock load in the drill string, while rotational torque is applied. Another is to introduce a lubricant, called a spotting fluid, into the zone of sticking within the bore hole. The intent of the spotting fluid is to consolidate the filter cake and provide lubrication. The drill string is then worked or cyclically loaded, in an attempt to break the sticking contact.
It would be extremely useful to be able to determine the stickometric properties of various filter cakes, in order to allow an optimization of formulation and technique to reduce the differential pressure sticking effect.
The consolidometric nature of the filter cake is also useful to know when comparing drilling and spotting fluid formulations, since this allows an estimate of the consolidation of the filter cake under the drill string while the string is stuck (for example during a fluid spotting procedure).
Further, it is of advantage to be able to off-centre the probe during the build-up of the filter cake, in order to determine what sort of unbalanced profile would be built up under similar conditions down the well. Lastly, it would be of advantage to be able to determine the filter cake thickness around the entire periphery of the permeable sleeve simulating the rock formation, and over a definite axial length, rather than only in a single location.